1. Field of the Invention
Aspects of this invention relate generally to suspension systems and to methods for reducing the transmission of motion in a certain bandwidth from a carrier to a payload, or the reduction in vibration in a certain bandwidth of a fixed carrier by its vibrating payload. More particularly, aspects of the current invention can be installed on a moving carrier that is subject to variations in acceleration as a result of its motion, where the variations experienced by the payload in a selected frequency band are damped.
2. Description of the Related Art
The mitigation of unwanted vibrations on motion-sensitive apparatus is a subject on which an abundance of information exists. In many cases, the purpose of such devices is to isolate a sensor or sensing apparatus from motions that would add noise or distort a measurement. Such motion isolation devices, which purpose is to damp the transmission of vibrations between a payload and a supporting carrier, typically use vibration absorbing materials, mechanical isolation involving springs and dampers, or pneumatic components and magnetic levitation in which the Lorentz force is employed. Many such systems employ a combination of active and passive feedback to adjust the effective mass, damping and stiffness of the system to optimize the isolation in order to suit the type of motion of the carrier and the mass of the payload being transported. A considerable part of the related art considers the isolation of sensors from the vibrations in the carriers the sensors are mounted on.
To understand the problem being solved by aspects of the current invention, an example of a motion isolation in one dimension is given in FIG. 1. The purpose of a motion isolating mechanism is to minimize the displacement xm of a mass m, acted on by force, fm transferred through the motion isolation apparatus from a force imparted by a carrier platform, fp, that is located at xp. Following Whorton et al (Damping mechanisms for microgravity vibration isolation, MSFC Centre Director's Discretionary Fund Final Report, Project No. 94-07, NASA/TM-1998-206953), if this force is transferred to the mass through a spring with constant, k, and damping, d, then:md2xmdt2+dd(xm−xp)/dt+k(xm−xp)=fm+fp  (1)which results in the following condition for force-free motion when the Laplace Transform is taken:ms2xm(s)+ds(xm(s)−xp(s))+k(xm(s)−xp(s))=0  (2)and which generates the following transfer function:xm(s)/xp(s)=(2ζωs+ω2)/(s2+2ζωs+ω2)  (3)where the natural frequency of the system, ω, is defined by ω2=k/m and the damping ratio ζ, is defined as ζ2=d2/4 km. The damping ratio is the ratio of the actual damping, d, to the damping required to reach critical damping, given by d2=4 km. The damping ratio must be less than 1 if the system is to oscillate.
This system acts as a low-pass filter that transfers carrier displacements below the damped natural frequency, ωd, where ω2d=ω2 (1−ζ2). An example of such a device is used in the Gedex airborne gravity gradiometer, where three passive translational isolation stages use springs and dampers reduce the accelerations experienced by the gradiometer at frequencies above 1 Hz.
Certain other approaches to vibration isolation increase the mass, m, so as to reduce the damped natural frequency. Such approaches are used in structural design to control the sway of buildings, but are not applicable for use in airborne vibration control applications as instruments and carriers with large masses become prohibitively expensive to operate. Others, such as in U.S. Pat. No. 5,549,270, lower the effective spring constant k by mechanical means but have limited travel. Yet other methods use acceleration feedback on the carrier force to modify the effective mass, velocity feedback to modify the damping, and displacement feedback to modify the effective spring constant. In so doing, the damping and damped natural frequency can be modified dynamically as required. Examples of such approaches are U.S. Pat. No. 6,196,514 and US Patent Application Publication No. 2002/0092350 in which a force feedback transducer is employed.
In the case of a geophysical measurement, mechanical vibrations can be introduced in mobile carriers through a number of pathways. For example, vibrations can be caused by unbalanced moving mechanical components, uneven friction as mechanical joints work against each other, buffeting of the carrier by wind or wave action in the case of an aircraft or ship, the motion of the carrier across uneven ground in the case of a land vehicle, the strumming of cables in the wind or water, or pendular motion in cases where a carrier is towed from a moving vehicle. Because the forces associated with carrier vibrations act on the sensors through their mounting points, these forces can result in torques being applied to the sensor. These torques, in turn, can cause the sensor to rotate relative to the mobile carrier upon which it is mounted. Such rotations are in superposition to the rotations imparted to the sensor directly by the rotation of the mobile carrier itself. In general, carrier vibrations result in translations and rotations of the sensor relative to the carrier on which it is mounted.
Vibrations can introduce noise into sensor data in several ways. For example, mechanical vibrations can introduce small geometrical changes in the shape of the sensor, altering its sensitivity to the quantity it is detecting. An example of such an effect would be the motion of one capacitive electrode relative to another causing a change in capacitance, the self motion within a coil changing its inductance, the relative motion of two coils, changing their mutual inductance and the motion of a lens relative to its focal plane, causing blurring of the detected image. Noise can also be introduced in a measurement by small motions in nearby objects, which if they are conductive could induce the flow of eddy currents and so add noise to an electromagnetic measurement, or to a measurement of a quality dependent on an electromagnetic characteristic for its resolution. Vibrations can also introduce small displacements in the positions of nearby objects, which could change the background field detected by the sensor, particularly if the object were electrically conductive, magnetically permeable or permanently magnetized in the case of an electromagnetic measurement, or which if sufficiently massive, would add noise to a gravity measurement.
Noise can also be introduced into the measurement of a vector component if the sensor rotates in a large background static field. Such considerations apply in particular to electric, magnetic, electromagnetic and gravimetric measurements where the static field of the Earth is much larger than the variations in field that are typically mapped. In order to reduce motion induced noise in the sensors caused by their rotations in the larger background field of the Earth, the sensors are often mounted on a sensor platform (“SP”) which is mechanically isolated from the mobile carrier with which it is transported. This isolation has two important components, a rotational isolation so that rotations of the vehicle are decoupled from the SP, and isolation from translational acceleration, noting that translational accelerations can also couple into rotational or angular accelerations if the forces are not applied through the centre of mass of the SP.
As discussed previously, various motion-isolation techniques have been applied to numerous combinations of carrier and payload. However, in the field of airborne electromagnetic measurement, suspension techniques have largely been limited to the use of vibration absorbing materials, including systems of bungee cords, to provide the spring constant and the damping elements such as U.S. Patent Application Publication No. 2003/0094952, U.S. Patent Application Publication No. 2003/0169045, and Canadian Patent No. 2722457. Techniques such as the ones used in the cited patents have been effective in suppressing higher frequency (25 Hz and above) vibrational noise, which is often referred to in the airborne electromagnetic survey industry as microphonic noise. However at frequencies below 25 Hz, vibration-induced noise usually overwhelms the signal being measured, providing an effective limit below which it is extremely difficult to acquire meaningful electromagnetic data at low signal levels. However, the band between 1 and 25 Hz is also one where a number of important phenomena, such as induced polarization, can be more easily observed if data of sufficiently low noise could be collected. Induced polarization measurements are an important tool for diagnosing the presence of certain mineral deposits and other geological phenomena. Limiting the acquisition band to 25 Hz and above also renders many highly conductive ores invisible to some airborne electromagnetic systems, and can limit the depth of exploration of electromagnetic systems, particularly over areas where the ground is highly conductive.
Turner et al (U.S. Pat. No. 6,369,573) recognizes the importance of reducing the rotation of a towed airborne vehicle for making SQUID magnetometer measurements in the Earth's field. Turner uses a combination of nested spheres, liquid, baffles, springs and dampers and claims to reduce the rotational motion of a payload in the data acquisition band, greater than 20 Hz. Henderson et al. (U.S. Pat. No. 5,117,695) uses coaxial cylinders with the inner cavity filled with a damping fluid, together with springs and dampers to protect single axis devices, such as an accelerometer, and is intended for space vehicle applications.
In the field of airborne geophysics, pneumatic motion isolation has principally been used in gravity and gravity gradiometry, mainly through the use of air bearings for example. A high-precision two-frame inertial platform and a gravimeter sensor for airborne application in which an air bearing gyro was modified and used to stabilize the system. However, airborne geophysical data can generally not be acquired with a pneumatically based motion isolation system that can be deployed in an airborne device.
Measurement of low-noise, low-frequency electromagnetic data on a mobile carrier has challenges not presented by other measurements. In airborne operations, these can require distancing the electromagnetic sensors from the aircraft to limit mechanical or electrical noise which can be from the motion of the airframe through the Earth's magnetic field, on-board power, induction within the airframe, or by electromagnetic transmissions and aircraft propulsion systems. A common solution to the aircraft noise problem is to mount the receiving apparatus in an enclosed carrier towed below a helicopter or fixed wing aircraft at some distance typically ranging from 30 to 80 meters. As a result, any motion isolating apparatus mounted in the carrier faces limitations on weight and size that can be safely deployed and reliably controlled. In the case of an electromagnetic survey, the motion controlling apparatus must also be electromagnetically quiet, placing additional restriction on how a vibration isolation apparatus can be built and operated. Such restrictions have so far prevented successful routine acquisition of low amplitude electromagnetic data in the 1-25 Hz frequency band.
In the current state-of-the-art, noise is often reduced in a sensor measurement by stacking or averaging when a repetitive signal is being processed. Stacking or averaging in effect blends the sensor output acquired over a specific time interval. If the sensor is mounted on a mobile carrier, it may be displaced as the carrier to which it is attached is moved. If the amount of displacement is significant, the fields measured by sensor may vary in accordance with the proximity of the sensor to the phenomena causing the field. The result is that a single stack may blend fields scattered from a plurality of causes in different proportions, thereby limiting the spatial resolution, or the ability to resolve the various causes from each other. An example of such an effect is encountered in geophysical survey systems that are deployed on mobile airborne survey carriers. In such cases, measurements are typically made along parallel traverse lines at a speed of approximately 30 meters/second in the case of a helicopter or 90 meters/second in the case of a fixed-wing aircraft. For a signal acquired at 1 Hz, the sensor may typically have traversed 30 meters (helicopter) or 90 meters (fixed wing) over one cycle. If a stack were to include 10 cycles, the extent of the travel incorporated in a single stack would be 300 meters in the case of a helicopter system and 900 meters in the case of a fixed wing system. In such cases, it may be difficult to resolve a targeted cause, such as an ore deposit or structure associated with an ore deposit, with dimensions that are much smaller than the distance spanned by a single stack. Additional difficulties may include variations in topography, overburden, and water saturation, the fields from which are also blended in the stack, further obscuring the signal of the targeted cause within the stack.
The current state of the art in the airborne EM method is exemplified by such systems as U.S. Patent Application No. 2008/0246484A1, U.S. Pat. No. 7,157,914, or U.S. Patent Application No. 2003/0094952. The noise generated by carrier motion in such systems increases significantly below the 25 Hz threshold, rendering high-precision, low noise electromagnetic measurements below the 25 Hz barrier impractical. As the electrical properties of many objects of interest within the ground are detectable only in the frequency band spanning 1 Hz to 25 Hz, they are undetectable with moving electromagnetic survey systems employing state-of-the-art vibration damping means.